1. Field of the Invention
The present invention relates to a reversible heat-sensitive recording material, especially to which causes superior color development and high stabilization of an image.
2. Description of Related Art
Generally, heat-sensitive recording materials are obtained by forming on a substrate a heat-sensitive recording layer composed mainly of a usually colorless or light-colored, electron-donating leuco dye and an electron-accepting developer. On heating with a thermal head, laser beams or the like, the leuco dye and the developer react with each other to give a printed image.
When an image is once formed in such heat-sensitive recording material, it is impossible to erase the image and newly form another image on the spot. Therefore, in order to record further information, there is no choice but to record the information in a part of the heat-sensitive recording material where an image has not been formed. Consequently, while the heat-sensitive recording material is limited in the area of its heat-sensitive recording portion, the amount of information that the material is capable of recording is limited.
On the contrary, there is known a reversible heat-sensitive recording material comprising a leuco dye and an agent used both as developer and tone reducer (it will be referred to as "color developing/reducing agent" in the following) which causes color development or achromatization of the leuco dye on heating. One example of such color developing/reducing agent would be an amphoteric compound having an acidic group capable of allowing the leuco dye to develop a color and a basic group capable of achromatizing the leuco dye thus colored. The color development or the achromatization is carried out by causing the color-developing action of the acidic group or the achromatization action of the basic group, respectively, by controlling the heat energy. This method, however, does not allow complete switching between the color development reaction and the achromatization reaction by virtue of heat energy control alone. Since the two reactions occur at a certain ratio at the same time, sufficient coloring density can not be obtained and complete achromatization can not be executed. Thus sufficient image contrast can not be attained. Another possible disadvantage is that the basic group has achromatizing action also on a colored portion at ordinary temperature, so that the color density of the colored portion is unavoidably decreased with the lapse of time.
On the other hand, other kinds of such color developing/reducing agents have been developed, in which one example would be a phenolic compound or a phthalic acid compound having a long chain alkyl group. However, there is not yet known an agent for both color development and tone reduction which is superior in its ability of coloring and stabilizing an image. On using a color developing/reducing agent which is capable of a high image stability, there would be only a small difference obtained between the color density and the achromatization density because when improving the color density, an insufficient achromatization should be expected. On the other hand, on using a color developing/reducing agent with a high coloring capability, it might happen that the image is erased while being preserved, especially in a humid atmosphere.